Alpha-Adrenergic Receptors in Rat Myocardium: Identification by Binding of [3H]Dihydroergocryptine

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[3H]Dihydroergocryptine ([3H]DHE) binds to sites in membranes derived from rat myocardium that have the characteristics expected of α-adrenergic receptors. The binding is saturable with 41 fmol [3H]DHE bound per mg of protein and of high affinity with KD = 2.9 niu. The binding is rapid and readily reversible. Adrenergic agonists compete with [3H]DHE for binding in the order: epinephrine > norepinephrine » isoproterenol; and adrenergic antagonists compete for binding in the order: phentolamine » propranolol. For comparison, (-)[3H]dihydroalprenolol [(-)[3H]DHA] was used to bind to sites in the same membrane preparations having characteristics of β-receptors. The number and affinity of β-receptors were quite similar to those of the α-receptors with 46 fmol (-)[3H]DHA per mg protein bound at saturation and KD = 2.5 nM. These techniques allowed identification of both ft- and o-adrenergic receptors in membranes derived from isolated atria, right ventricular free walls, and left ventricles including interventricular septa. This is the first report documenting direct identification of myocardial a-receptors by radioligand-binding techniques and complements the literature previously reporting myocardial inotopic and electrophysiological responses to α-adrenergic stimulation.

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